This invention relates generally to noise-attenuating devices, and more particularly provides novel apparatus and methods for silencing vortex "whistle" noises generated within the radial-to-axial intake section of the load compressor of a gas turbine engine auxiliary power unit.
In addition to their traditional propulsion functions, gas turbine engines are also often used as auxiliary power units (APU's) to supply pneumatic power to a wide variety of accessory devices and systems. This is accomplished by bleeding a desired quantity of compressed air from a centrifugal "load" compressor which is connected to and driven by the engine's drive shaft.
Ambient air is drawn axially into the load compressor through the annular flow passage of an intake assembly which has a circular, radially outwardly facing inlet opening that circumscribes the drive shaft. Adjustable inlet guide vanes are mounted in a mutually spaced relationship around the circumference of the radial inlet opening for conjoint pivotal motion about axes prallel to the shaft axis between a fully closed position in which the vanes are each generally tangentially disposed relative to their inlet opening, and a fully open position in which each of the vanes extends generally radially inwardly therefrom. By selectively adjusting the angular position of these vanes the flow rate of air entering the load compressor (and thus the flow rate of compressed air supplied to the pneumatically-operated accessory apparatus) during engine operation may be accurately regulated.
Because of their orientation relative to the drive shaft axis, the inlet guide vanes, within a certain range of opening angles, impart to air traversing the intake assembly flow passage a desirable vortex pattern in which the air swirls about the shaft axis as it is drawn axially into the load compressor. This vortex pattern causes the air therein to contact the curved impeller blades of the centrifugal load compressor at an efficient angle of incidence.
However, in conventional radial-to-axial air intake assembles of the type described, the induced air swirl also creates (within certain ranges of air flow rates and vane opening angles) a shrill intake noise known as vortex "whistle". Vortex whistle is undesirable from two standpoints. First, it is often unacceptable under applicable acoustic standards. Secondly, generation of the whistle within the intake assembly causes a concomitant aerodynamic energy loss which diminishes the efficiency of the load compressor.
Accordingly, it is an object of the present invention to provide apparatus and methods for eliminating or minimizing intake vortex whistle in gas-utilizing devices such as the described load compressor.